USMLE Road Map Physiology – easy-to-follow hierarchical outline format guides students through the most important aspects of each discipline.
I. Plasma Membrane
A. The structure of the plasma membrane allows the separation and creation of distinct molecular environments within cells. The lipid bilayer is similar to thin layers of oil surrounding fluid ozone. Thus, the lipid bilayer divides the cell into functional compartments.
B. The fluid mosaic model is the accepted view of the molecular nature of plasma membranes.
- The model proposes that proteins traverse the lipid bilayer and are incorporated within the lipids.
- Proteins and lipids can move freely in the plane of the membrane, producing the fluid nature of the membrane.
C. The plasma membrane is composed of phospholipids and proteins.
- Membrane lipids can be classified into three major classes: phospholipids, sphingolipids, and cholesterol.
a. Phospholipids are the most abundant membrane lipids. (1) They have a bipolar (amphipathic) nature, containing a charged head group and two hydrophobic (water-insoluble, noncharged) tails. (2) The hydrophobic tails face each other, forming a bilayer and exposing the polar head group to the aqueous environment on either side of the membrane.
b. Sphingolipids have an amphipathic structure similar to phospholipids that allows them to insert into membranes. These lipids can be modified by the addition of carbohydrate units at their polar end, creating glycosphingolipids in brain cells.
c. Cholesterol is the predominant sterol (unsaturated alcohols found in animal and plant tissues) in human cells; it increases the fluidity of the membrane by inserting itself between phospholipids, improving membrane stability.
The accumulation of glycosphingolipid associated with Tay-Sachs disease causes paralysis and impairment of mental function.
- Membrane proteins that span the lipid bilayer are known as integral membrane proteins, whereas those associated with either the inner or the outer surface of the plasma membrane are known, respectively, as peripheral or
- lipid-anchored membrane proteins.
- a. The majority of integral membrane proteins span the bilayer through the formation of -helices, a group of 20–25 amino acids twisted to expose the hydrophobic portion of the amino acids to the lipid environment in the membrane (Figure 1–1).
- b. Protein content of membranes varies from less than 20% for myelin, a substance that helps the propagation of action potentials, to more than 60% in liver cells, which perform metabolic activities.
- c. Cellular proteins act as receptor sites for antibodies as well as hormone-, neurotransmitter-, and drug-binding sites.
- d. Enzymes bound to the cell membrane are often involved in phosphorylation of metabolic intermediates.
- e. Carrier proteins in the membrane transport materials across the cell membrane.
- f. Membrane channels allow polar charged ions (Na+ , K+ , Cl− , and Ca2+) to flow across the plasma membrane. Ion channel gates regulate ion passage and are controlled by voltage (voltage gated), ligands (ligand gated), or mechanical means (mechanically gated).